Transformer is a frequently used electronic element in various types of electric equipment. FIG. 1 illustrates the structure of a conventional transformer 1 which includes a primary coil 10 and a secondary coil 11 spaced from the primary coil 10. The primary coil 10 and the secondary coil 11 generate electromagnetic coupling effect through an iron core 12 to transform voltage.
The primary coil 10 and the secondary coil 11 of the transformer 1 mentioned above are adjacent to each other, as a result the leakage inductance of the transformer 1 is higher that results in a greater energy loss during voltage transformation. To remedy this problem another type of transformer 2 has been developed as shown in FIG. 2. It adopts a three-layer winding structure (or called the transformer with sandwich winding). It has an upper layer, a middle layer and a lower layer to become respectively a first primary coil 20, a secondary coil 21 and a second primary coil 22. The primary coils 20 and 22 are not adjacent to the secondary coil 21, the leakage inductance is much smaller, and energy loss also is smaller.
However, when the two types of transformers previously discussed are coupled with an asymmetrical half bridge oscillation circuit, the leakage inductance is too large or too small to provide the leakage inductance needed by the asymmetrical half bridge oscillation circuit. Hence how to provide a transformer capable of controlling leakage inductance is an issue remained to be resolved in the industry.